Thirtieth
来自Big Physics
wiktionary
From Middle English threttiþe, thrittyde; equivalent to thirty + -eth.
etymonline
thirtieth (adj., n.)
"next in order after the twenty-ninth; an ordinal numeral; being one of thirty equal parts into which a whole is regarded as divided;" Middle English threttyth, from Old English þritigoða; see thirty + -th (1). Respelled 16c. to conform to new spelling of thirty.
